The invention relates to telecommunications, and more particularly, to a digital subscriber line and POTS voice interface between a telephone line and central office equipment of a network operator providing POTS voice and digital subscriber line data services.
A conventional telephone transmission line is typically comprised of a pair of copper conductors that connect a telephone set to the nearest central office, digital loop carrier equipment, remote switching unit or any other equipment serving as the extension of the services provided by the central office. This pair of copper conductors, which is also referred to as a twisted pair, has its leads named as tip and ring. The tip and ring nomenclature is derived from the electrical contacts of an old-style telephone plug. A number of such twisted pairs are generally bundled together within the same cable binder group.
The demand for high bandwidth data transmission over existing telephone transmission lines has led to the development of digital subscriber line (DSL) technology. Several variations of DSL technology (referred to generically as xDSL or simply DSL) are evolving, such as SHDSL (symmetric high-bit-rate DSL), HDSL2 (second-generation high-bit-rate DSL), RADSL (rate adaptive DSL), VDSL (very high-bit-rate DSL), and ADSL (asymmetric DSL). In general, a digital subscriber line is comprised of two DSL modems coupled to one another by a twisted pair. The transmit (Tx) and receive (Rx) signals of DSL communications are carried by the twisted pair.
Some DSL technologies, such as ADSL, have the advantage that ordinary voice data transmissions (referred to as Plain Old Telephone Service or POTS) can share the same telephone line with digital data transmissions. The lower frequency band of the telephone line is used for voice data, while the upper frequency bands are used for digital data. However, because each of these frequency bands operates on the same phone line, such DSL technology employs a splitter to isolate the signals of each band. More specifically, the splitter isolates the low-frequency components (POTS data) of the transmission, and the high-frequency components (digital data) of the transmission. The splitter also operates as a mixer to combine the high-frequency digital data with the low-frequency voice data, and provides the combined signal to the telephone line.
A problem with this splitter approach is that the passive elements (e.g., inductors and capacitors) that can be used to implement the low and high pass filters of the splitter are bulky and cumbersome. This added bulk requires more physical space to house the splitter, and consequently occupies more physical space in the central office of the telephone company. In the aggregate, the physical space consumption resulting from this approach is costly. Also, this approach is associated with additional material and manufacturing costs involved in building the splitters, as well as costs associated with deploying those splitters.
A silicon broadband subscriber line interface circuit (SLIC), on the other hand, provides an active discrete splitter that is more compact than a passive splitter. However, such technology requires a high-voltage, high-speed silicon process and consumes significantly more power thereby limiting the service circuit line density. Moreover, this approach requires an additional power backup in order to sustain the supported communication channels for the same time period compared to that of a POTS only service in the event of a power failure.
What is needed, therefore, is an improved interface between a telephone line and the central office that can receive and isolate (or combine) low frequency POTS data and high frequency digital data from the telephone line.
One embodiment of the present invention provides a technique for synthesizing a negative impedance to provide a desired input impedance of a POTS line card, the POTS line card having a DC blocking capacitor of a DSL coupling transformer connected across its two wire interface. The technique includes providing a transmit line output signal of a SLIC included in the POTS line card to a receive line input of the SLIC by way of an impedance. The technique further includes phase inverting the transmit line output signal thereby effectively generating a negative impedance. The technique further includes muting the negative impedance over DSL frequency band signals, but not over POTS frequency band signals. Another embodiment of the present invention provides a negative impedance synthesis circuit operatively coupled between a transmit line output of a POTS line card SLIC and a receive line input of the POTS line card SLIC. The circuit includes an impedance in series with a unity gain phase inverter. The unity gain phase inverter is adapted to receive a transmit line output signal of the POTS line card SLIC, and to phase invert the transmit line output signal thereby synthesizing a negative impedance. The circuit further includes a low pass filter in series with the unity gain phase inverter. The low pass filter is adapted to mute the synthesized negative impedance over DSL frequency band signals, but not over POTS frequency band signals.